Radio capsule battery

ABSTRACT

A battery for an active radio capsule wherein one of the electrodes, a silver-silver chloride electrode, functions as the reference electrode for a sensor by means of an electrolytic path from the battery chamber to the solution external to the capsule. The reference electrode, which is stable and insensitive to foreign ions, is disposed in a closed chamber filled with an electrolyte which is immobilized by an absorptive powder. One or more fibrous plugs form an electrical connection between the chamber and the surrounding medium. The reference electrode is designed to have a low, stable internal impedance and has a high current capability.

Waited States Eatent n91 Norby et a1.

[4 1 Feb. 12, 1974 1 1 RADIO CAPSULE BATTERY [76] Inventors: Thomas E.Norby, 3617 Allendale Dr.; Charles H. Rogers, 3724 Arrowhead Dr., bothof Raleigh,

[22] Filed: June 30, 1971 [21] Appl. No.: 158,294

[52] US. Cl 128/2 1?, 136/177 [51] int. Cl A611) 5/07 [58] Field ofSearch 128/2 P, 2 R, 2.1 R, 419 R;

[56] References Cited UNITED STATES PATENTS 3,133,537 5/1964 Muth 128/2P 3,033,911 5/1962 Duddy 136/177 3,190,285 6/1965 Muth 128/2 P FOREIGNPATENTS OR APPLICATIONS 1,202,435 10/1965 Germany '128/2 P PrimaryExaminerWilliam E. Kamm- Attorney, Agent, or Firm-William J. Simmons,Jr.

[5 7 ABSTRACT A battery for an active radio capsule wherein one of 14Claims, 3 Drawing Figures PAFENIED FEB 1 21914 COIL ' INVEN TORS.

mas E. Norby Charles H. Rogers 1 RADIO CAPSULE BATTERY BACKGROUND OF THEINVENTION This invention relates to a radio capsule of the type utilizedin a telemetering system for transmitting physi' ological informationfrom within the human body, and more particularly, it relates to animproved battery for an active radio capsule.

Telemetering systems for transmitting information such as temperature,potential difference, pressure and specific ion activity such as pH, pKand the like include a radio capsule which can be swallowed by apatient. Conventional active radio capsules comprise a sensor ortransducer, a power supply and a modulator and transmitter. Due to thenature of the use to which a radio capsule is put, i.e., it is swallowedby a patient and transmits information from within the gastrointestinaltract, some severe design limitations are placed on the componentsthereof. The number of components must be kept to a minimum due to thesmall space available, the sensor, battery and transmitter beingpackaged within a housing having a length of about three-fourths in. anda diameter of about five-sixteenths in. Also inert materials must beused to avoid injury to the human body.

A radio capsule power source should provide a stable voltage supply tothe transmitter during its entire lifetime. However, due to thelimitations imposed on size and materials, capsule batteries aregenerally of the type that provide a low voltage which decreases withusage. Although the volume of the power supply should be as small aspossible, some capsule batteries, the volume of which are more than 40percent of the entire capsule, have been developed in an attempt toprevent voltage drift. Such batteries have not been acceptable sincethey increase the size of the capsule beyond acceptable limits.

Other capsules have included discrete miniature batteries such asmercury cells, but dangers arising from toxic chemical substances cannotbe fully avoided by encapsulation of the battery. Leakage due toselfcorrosion and due to biting on capsules must be considered.Non-toxic batteries are disclosed in US. Pat. Nos. 3,340,886 and3,323,513, issued to H. J. Noller and M. Gnadke, respectively. Poorvoltage stability exists in these batteries since the electrodes thereofare wetted by ambient gastric or intestinal fluid which acts as theelectrolyte. Capsule batteries having a self-contained electrolyte aredisclosed in US. Pat. Nos. 3,133,537 and 3,190,285 issued to H. Muth.Due to the type of electrode construction and the type of semi-permeableconnection between the battery electrolyte and the fluid outside thecapsule, the voltage produced by the Muth batteries is not adequatelystabilized.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to provide a small, stable, radio capsule battery, oneelectrode of which functions as a reference electrode for a sensor.Another object is to provide a radio capsule battery wherein thereference electrode has a stable internal impedance and a relativelyhigh current capability. Still another object is to provide a radiocapsule battery having a silver-silver chloride reference .electrodewhich exhibits uniform physical and electrical characteristics during arelatively long period of battery usage.

Briefly, the battery of the present invention is of the type that isadapted to be used in a radiov capsule for. investigating a condition ofa region that is readily accessible only by wireless probes. Suchcapsules generally comprise a sensor for providing a voltage, the valueof which is determined by the investigated condition. An oscillator inthe capsule generates an rf signal, the frequency of which is determinedby the sensor voltage. The radio capsule battery is adapted to beactivated by an electrolyte prior to use, such batteries having twoelectrodes for providing an operating voltage for the oscillator, one ofthe electrodes also functioning as the reference electrode for thesensor. The battery of the present invention is characterized in that itcomprises means defining a chamber in the capsule and means defining atleast one hole in the chamber defining means, a mass of low porositymaterial being disposed in the hole for permitting only an extremelylimited diffusion of electrolyte to or from the chamber. One of thebattery electrodes is a consumable electrode, and the other of theelectrodes comprises a silver portion and a silver chloride portioncontacting the silver portion, a part of the silver portion beingexposed so that it is free to contact the electrolyte. That portion ofthe silver chloride which contacts the silver is thin in cross sectionso that the impedance of the electrode is relatively low.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view of aradio capsule which incorporates the battery of the present invention.

FIG. 2 is an enlarged, front view of the silver-silver chlorideelectrode, the side view of which is shown in FIG. 1.

FIG. 3 is an oblique view of another silvensilver chloride electrodethat can be utilized in the battery of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The radio capsule illustratedin FIG. 1 comprises a battery chamber 1 1 a sensing electrode 12 and anoscillator circuit 13, oscillator coil 14 functioning as thetransmitting antenna. A description of ion sensing electrode 12 can befound in copending' application Ser. No. 158,293, entitled GlassElectrode Structure for Radio Capsules, filed on even date herewith,which discloses an electrode structure that is accurate and stable overlong periods of time.

The battery chamber 11 comprises a cylindrically shaped side wall 16 andelastic cap 17 made of urethane, viton rubber, or the like and a wall 18which separates the battery from the circuit 13. The walls 16 and 18 maybe made of a non-conductive material such as epoxy, plastic, or thelike. A silver-silver chloride electrode 19 and a consumable electrode20 made of zinc, magnesium or the like are disposed in chamber 11.Circuit 13 comprises a plurality of discrete components arranged in aso-called cord wood fashion between two disk shaped circuit boards 22and 23. Zinc electrode 20, which may be in the form of a bent wire orplate is affixed to one side of circuit board 22, whereas silver wire24, which forms a part of electrode 19 extends through circuit board 22and contacts circuit board 23 and forms an electrical connection betweenthese two 3 circuit boards. ion sensitive electrode 12 is connected tothe oscillator circuit by a lead 26.

It is well known that one of the battery electrodes may be used as thereference electrode for the sensor circuit. The advantageous propertiesof an accurate and stable sensor can only be fully realized if areference electrode of comparable stability and insensitivity to foreignions is employed. in accordance with the present invention, both thedesign of the silver-silver chloride reference electrode 19 and theexternal path between the reference electrode and the ion sensingelectrode structure 12 are designed to provide a stable referencevoltage for the ion sensing circuit. Reference electrode 19 is disposedin a closed electrolyte chamber 11 in which the electrolyte, which mayconsist of a -10 percent sodium chloride solution, is immobilized by anabsorptive powder 28, i.e. powder 28 prevents varying exposure ofbattery parts when the capsule changes orientation and a smallelectrolyte void is present. For the sake of clarity only a portion ofthe absorptive powder is illustrated. The absorptive powder may consistof grains of synthetically produced crystalline metal alumino-silicatesthat have been activated by removing their water of hydration. Thesematerials are crystalline zeolites having a basic formula of ll TQQG AlOQ- xSiOQ who where M is a cation of n valence. A preferred type ofabsorptive powder was prepared by crushing commerically availablepellets of an aluminum sodium silicate. The crushed material was gradedto size by passing it over suitable 3O screens. All pieces which passedthrough a 0.025 in. mesh but failed to pass through a 0.015 in. meshwere used. in addition to providing a porous absorbing medium for theelectrolyte, absorptive powder 28 reduces the chance of a large bubbleforming and lodg-, ing in the chamber 11 when the battery is activatedand also prevents separation of the electrolyte from the sonsumableelectrode 20 if hydrogen is evolved.

One or more fibrous plugs 30 are disposed in holes 31 to form a wettedelectrical path or connection between the electrolyte in the chamber 11and the surrounding medium. The electrical path through the fibrousplugs must remain conductive in a variety of unusual media such asstomach juice, food, commercial anticids and digestive materials. Itmust also remain conductive without the aid ofa positive flushing actionwhich is used in some liquid junctions. Regardless of the differentionic concentrations which may exist in the electrolyte and in the mediain which the radio capsule is disposed, the potential across the fibrousplugs should remain so small that it is insignificant. Fibrous junctionsor plugs in which the fibers were oriented parallel to the direction offlow were best suited for this application. Such an orientation, whereinthe fibers are substantially parallel to the axis of the hole permits atleast some of the fibers to extend from chamber 11 to that region whichis external to the capsule. While providing an electrical connectionbetween the electrolyte and the surrounding media, the fibrous plugs canstill be of low porosity, thus limiting the diffusion of materials fromthe electrolyte within chamber 11 to the surrounding media or viceversa. A good electrical connection was obtained by using a single birchwood cylinder 0.03 in. in diameter and 0.05 in. long, the fibers thereofbeing parallel to the longitudinal axis of the cylinder. Although wood,asbestos. and porous ceramic plugs have been used, wood is the preferredmaterial.

In a self-contained active radio capsule it is convenient to combine thereference electrode of the sensing circuit with one electrode of theinternal battery. in such a situation, however, the reference electrodemay be required to carry much more current than conventional electrodesare designed to supply. For example, in a radio capsule designs for theremote readout of pH, the reference electrode was required to carry asmuch as 10 ampere, a factor of 10 more than conventional referenceelectrodes are designed to supply. Since the accuracy of the measuredion activity depends on the stability of the reference potential, it isimportant that the reference electrode maintain a nearly constantpotential while supplying 10 ampere to operate the transmittingcircuitry. There are three important factors in the design of areference electrode having high current capability. First, the electrodeshould have a stable internal impedance. Secondly, the surroundingelectrolyte should not change in its chloride ion concentration withtime. Finally, the interface between the silver and the silver chlorideshould exhibit uniform physical electrical characteristics as the silverchloride is reduced to silver.

A stable internal impedance was obtained by keeping the internalimpedance as low as possible. A low internal impedance was maintained bycausing the metallic silver portion of the electrode to directly contactthe electrolyte and by keeping the silver chloride thin in cross-sectionin that portion thereof which contacts the silver electrode portion. 1fthe silver chloride is too thick at the silver-silver chlorideinterface, an undesirable voltage drop will exist since the silverchloride is an insulator. A preferred electrode embodiment having thesecharacteristics is illustrated in FIGS. 1 and 2. The portion of thesilver wire 24 extending beyond wall 18 into chamber 11 consists of aportion 32 which is bent in a first plane and a portion 34 which is bentin a second plane to form a loop or closed figure such as therectangular shape illustrated. The volume within loop 34 and portion 32is filled with silver chloride which may be applied to the silver wireby inserting the wire into a pool of molten silver chloride. As thechemical reaction proceeds during battery usage, wherein the silverchloride is reduced to silver, the physical area of the interfacebetween the silver and silver chloride remains nearly constant andelectrically similar to the originally formed interface. Although thereis an i.r. voltage drop near the silver chloride, it is substantiallyconstant.

FIG. 3 illustrates another design which may be utilized for thesilver-silver chloride electrode. in this embodiment the interfacebetween the silver and silver chloride is formed by melting a uniformcoating of silver chloride 33 onto a piece of silver foil 40. This canbe done by applying a thin layer of powdered silver chloride to thesurface of foil'40 and thereafter fusing the particles. The foil 40which may be U-shaped as i1- lustrated in FIG. 3, is bonded to a silverwire 42 by soldering or the like. In this embodiment the internalimpedance is made small by causing the metallic silver to directlycontact the electrolyte and by keeping the silver chloride thin in crosssection. Also, the physical area of the interface between the silver andsilver chloride remains substantially constant as the chemical reactionreduces the silver chloride to silver during battery operation.

Since the silver chloride evolves chloride ions at a rate proportionalto the current being supplied by the battery, it is necessary to eitheractively control the concentration of the electrolyte or begin with avolume which is not appreciably affected by the release of thesechloride ions. In a battery having an electrolyte volume of 7.2 X cu.in. the chloride ion concentration was not appreciably affected by therelease of chloride ions. Moreover, the use of the wooden plugs 30 tomaintain a liquid interface between the battery chamber and the mediabeing measured did not permit a rapid diffusion of ions between thebattery electrolyte and the surrounding medium. Thus the chloride ionconcentration of the electrolyte did not appreciably vary during use. A5-10 percent sodium chloride solution provided best electrodeperformance. At this concentration level, initial stable potentials wereestablished at a satisfactory rate, and in the time period of capsuleoperation desired, i.e. 18 hours, the most stable reference potentialscould be maintained.

The battery chamber lll may be filled with porous powder and electrolyteso that the capsule can be tested. Thereafter, the chamber 11 can beemptied of absorptive powder and electrolyte, and the capsule can bestored for ultimate use. To activate the battery, the user addsabsorptive powder and electrolyte to the chamber ill and affixes cap 37.Alternatively, the absorptive powder could be stored in chamber ll whilethe capsule is on the shelf. The user could inject electrolyte throughcap 17 by means of a syringe. The battery described herein has anindefinite shelf life and has an operating lifetime of at least 50hours.

A battery containing radio capsule has been described herein as beingcapable of measuring hydrogen ion activity in the human alimentarycanal. Such capsules can also be used to determine specific ion activityand other information in animals or inanimate objects such as chemicalreaction or storage vessels, where the need to fully enclose the systemfor purposes of sterility or confinement of reaction products because ofpoisonous gas generation, presence of radio active materials, and thelike, makes a wireless telemetry system advantageous.

We claim:

1. In a radio capsule for investigating a condition of a region that isreadily accessible only by wireless probes, said capsule being of thetype comprising a sensor for providing a voltage, the value of which isdetermined by said investigated condition,

an oscillator for generating an rf signal modulated by said sensorvoltage, and

a battery adapted to be activated by an electrolyte prior to use, saidbattery having two electrodes for providing an operating voltage forsaid oscillator, one of said electrodes also functioning as thereference electrode for said sensor, said battery being characterized inthat it comprises means defining a chamber in said capsule, said twoelectrodes being disposed'in said chamber,

means defining at least one hole in said chamber defining means,

a plug of fibrous material disposed in said hole for permitting only anextremely limited diffusion of electrolyte to or from said chamber, thefibers of 6 one of said electrodes being a consumable electrode,

and

the other of said electrodes comprising a silver portion and a silverchloride portion contacting said silver portion, a part of said silverportion being exposed so that it is free to contact said electrolyte,that portion of said silver chloride which contacts said silver beingthin in cross section so that the impedance of the electrode isrelatively low.

2. A radio capsule in accordance with claim 1 wherein said fibrousmaterial is wood.

3. A radio capsule in accordance with claim 2 wherein said chamber issubstantially filled with a porous powder, said electrolyte beingdisposed within the voids between adjacent grains of said powder andwithin the pores thereof.

4. A radio capsule in accordance with claim 3 wherein said other of saidelectrodes comprises a silver wire which is bent at one end in the formof a substantially closed loop, said silver chloride filling the spacewithin said loop and contacting that portion of said silver wiredisposed on the inner surface of said loop.

5. A radio capsule in accordance with claim 4 wherein said loopisdisposed in a first plane, thatpon tion of said silver wire immediatelyadjacent to said loop being bent in a U-shaped manner in a second planewhich intersects said first plane the volume within said loop and saidU-shaped portion being filled with said silver chloride.

6. A radio capsule in accordance with claim 3 wherein said other of saidelectrodes comprises a sheet of silver foil, a thin layer of silverchloride disposed on one surface of said silver foil, and a silver wirebonde to said silver foil.

7. A radio capsule in accordance with claim 6 wherein said silverfoil-silver chloride composite is U- shaped. 1

8. A radio capsule in accordance with claim 1 wherein said chamber hasan open end, said capsule further comprising an elastic cap disposedover said open end. i

9. A radio capsule in accordance with claim 1 wherein said other of saidelectrodes comprises a silver wire which is bent at one end in the formof a substantially closed loop, said silver chloride filling the spacewithin said loop and contacting that portion of said silver wiredisposed in the inner surface of said loop.

10. A radio capsule in accordance with claim 9 wherein said loop isdisposed in a first plane, that portion of said silver wire immediatelyadjacent to said loop being bent in a U-shaped manner in a second planewhich intersects said first plane the volume within said loop and saidU-shaped portion being filled with said silver chloride.

1 l, in a radio capsule for investigating a condition of a region thatis readily accessible only by wireless probes, said capsule being of thetype comprising a sensor for providing a voltage, the value of which isdetermined by said investigated condition,

an oscillator for generating an rf signal modulated by said sensorvoltage, and

a battery adapted to be activated by an electrolyte prior to use, saidbattery having two electrodes for providing an operating voltage forsaid oscillator, one of said electrodes also functioning as thereference electrode for said sensor,

said battery being characterized in that it comprises posed so that itis free to contact said electrolyte, that portion of said silverchloride which contacts said silver being thin in cross section so thatthe impedance of the electrode is relatively low. 12. A radio capsule inaccordance with claim 11 wherein said porous powder is a crystallinezeolite.

13. A radio capsule in accordance with claim 12 wherein the voidsbetween the grains of porous powder and the pores thereof are filledwith an electrolyte.

14. A radio capsule in accordance with claim 13 wherein said electrolyteconsists of a 5 to 10 sodium chloride solution.

2. A radio capsule in accordance with claim 1 wherein said fibrousmaterial is wood.
 3. A radio capsule in accordance with claim 2 whereinsaid chamber is substantially filled with a porous powder, saidelectrolyte being disposed within the voids between adjacent grains ofsaid powder and within the pores thereof.
 4. A radio capsule inaccordance with claim 3 wherein said other of said electrodes comprisesa silver wire which is bent at one end in the form of a substantiallyclosed loop, said silver chloride filling the space within said loop andcontacting that portion of said silver wire disposed on the innersurface of said loop.
 5. A radio capsule in accordance with claim 4wherein said loop is disposed in a first plane, that portion of saidsilver wire immediately adjacent to said loop being bent in a U-shapedmanner in a second plane which intersects said first plane the volumewithin said loop and said U-shaped portion being filled with said silverchloride.
 6. A radio capsule in accordance with claim 3 wherein saidother of said electrodes comprises a sheet of silver foil, a thin layerof silver chloride disposed on one surface of said siLver foil, and asilver wire bonded to said silver foil.
 7. A radio capsule in accordancewith claim 6 wherein said silver foil-silver chloride composite isU-shaped.
 8. A radio capsule in accordance with claim 1 wherein saidchamber has an open end, said capsule further comprising an elastic capdisposed over said open end.
 9. A radio capsule in accordance with claim1 wherein said other of said electrodes comprises a silver wire which isbent at one end in the form of a substantially closed loop, said silverchloride filling the space within said loop and contacting that portionof said silver wire disposed in the inner surface of said loop.
 10. Aradio capsule in accordance with claim 9 wherein said loop is disposedin a first plane, that portion of said silver wire immediately adjacentto said loop being bent in a U-shaped manner in a second plane whichintersects said first plane the volume within said loop and saidU-shaped portion being filled with said silver chloride.
 11. In a radiocapsule for investigating a condition of a region that is readilyaccessible only by wireless probes, said capsule being of the typecomprising a sensor for providing a voltage, the value of which isdetermined by said investigated condition, an oscillator for generatingan rf signal modulated by said sensor voltage, and a battery adapted tobe activated by an electrolyte prior to use, said battery having twoelectrodes for providing an operating voltage for said oscillator, oneof said electrodes also functioning as the reference electrode for saidsensor, said battery being characterized in that it comprises meansdefining a chamber in said capsule, said two electrodes being disposedin said chamber, said chamber being substantially filled with a porouspowder, means defining at least one hole in said chamber defining means,means having low porosity disposed in said hole for permitting only anextremely limited diffusion of electrolyte to or from said chamber, oneof said electrodes being a consumable electrode, and the other of saidelectrodes comprising a silver portion and a silver chloride portioncontacting said silver portion, a part of said silver portion beingexposed so that it is free to contact said electrolyte, that portion ofsaid silver chloride which contacts said silver being thin in crosssection so that the impedance of the electrode is relatively low.
 12. Aradio capsule in accordance with claim 11 wherein said porous powder isa crystalline zeolite.
 13. A radio capsule in accordance with claim 12wherein the voids between the grains of porous powder and the poresthereof are filled with an electrolyte.
 14. A radio capsule inaccordance with claim 13 wherein said electrolyte consists of a 5 to 10sodium chloride solution.